Residual current protection in variable-speed drives

Circutor | 26 de May de 2022

More and more industrial processes these days rely on the use of one or more variable-speed drives: from basic applications such as pumping, to machines with precise robotic operations. But they are also used in everyday processes: modern washing machines, lifts and HVAC systems all use them.

aplicaciones protección industrial

Type B earth leakage protection must be installed in any installation where inverters are present; industry, assembly lines, forklifts, etc.

A variable-speed drive is a unique load, which brings into play specific considerations when choosing the proper residual current protection for it, especially in those processes and applications where continuity of service is essential. Poor design or bad engineering when choosing the residual current protection in these devices can lead to unexpected trips that can hinder a production process. Not to mention the direct cost of stopping and later starting up said process.

How to choose residual current protection

Some of the keys to understanding the kind of residual current protection to choose for your variable-speed drive lie in two of the specific features of how these drives operate:

1- The EMI filters commonly used by the drives (which attenuate the switching frequencies of the drive itself as it operates).

2- How their use disturbs the electrical grid through the presence of harmonics.

How does the EMI filter affect residual current protection?

It is important to know that all EMI filters behave like a grounded capacitor; therefore, when in use, they induce a ground current that any residual current device (RCD) will identify as a leak. If this ground current falls within the trip threshold of the RCD, it could actuate it.

According to standard IEC-60947-2-M, an RCD must trip in the presence of a current between 50% and 100% of its sensitivity. Let's consider some numbers to better understand this concept: if your RCD has a sensitivity of 300 mA, this means that it must trip if it measures a leakage of between 150 mA (50%) and 300 mA (100%).

For example, if we consider the typical maximum leakage current for three-phase, 1 KW, 10 mA variable-speed drives, approximately (the exact figures for your drive are available in its EMI filter data sheet), we recommend installing a maximum of 12 converters per RCD. Note that 12 x 10 mA = 120 mA, which is still well below the 150 mA that could cause a trip. This way, you will avoid nuisance trips from your RCD.

The second feature to take into account when choosing the RCD, if you want to properly protect the variable-speed drives, is that their use disturbs the electrical network through harmonics. The problem here is that some types of RCD are not designed for these harmonic frequencies and can induce new nuisance trips.

CIRCUTOR's products

CIRCUTOR, which has extensive experience designing residual current protection solutions since its founding in 1973, incorporates an ultra-immunized system into its type A protection systems that offers a series of advantages specifically designed for this field of application.

Specifically, all CIRCUTOR residual current protection devices guarantee tripping at or above 85% of their sensitivity, improving on the 50% required by the standard. This way, you can set the trip point at the top of the threshold, which in practice means you can connect an additional load. Either that, or you have a higher safety margin before tripping. In short: this is a very useful feature that helps cut down on nuisance protective trips.

Some of our differential protection equipment with ultra-immunised system


RGU-10, Electronic residual current protection relay of the WG series, Type A, 3 modules and display.

Reles diferencial para Transformador WGC, tipo A, 2 módulos y display

RGU-2, Electronic residual current protection relay of the WG series, Type A, 2 modules and display.

RGE-RL, relé diferencial para Transformador WGC, tipo A ultrainmunizados

RGE-RL, Differential Relay for WGC Transformer, Type A ultra-immunised.

WRU-10, Relé diferencial tipo A ultrainmunizados con transformador incorporado

WRU-10, Ultra-immunised type A differential relay with built-in transformer.

WGC Transformador diferencial

WGC, Differential transformer.

Another of the advantages of CIRCUTOR's Ultraimmunised system is the immunity it offers against the presence of harmonics, treating the measured current and eliminating or filtering frequencies above 50/60 Hz. Together with immunity to transients, this is a good feature that eliminates the trips caused in installations by this type of disturbance.

Trigger range between
85% - 100% of sensitivity.

Frequency response with leakage filtering at high frequencies.

More immunity to mains transistors up to 3 kA
for 8/20 μs pulses.

Moreover, by adding a display and/or communications to the ultra-immunized system's features, you can monitor and track any changes in the leakage current in the system you are protecting. This lets you anticipate any problems that may be caused by insulation breakdown and leakage currents.

The next time you have to choose the protection device for variable-speed drives, keep these basic tips in mind. And remember that CIRCUTOR has a wide range of residual current protection solutions that, with their immunity and communications features, provide the ideal solution for this widely-used application.

Choosing the right residual current protection is key to putting an end to nuisance trips in your installation. And it's something you can do with our ultra-immunized system.